Patent Literatures 1 and 2, etc., describe poppet valves each including a head portion formed integrally with a stem end portion. A poppet valve to be used in an internal combustion drives an engine by opening and closing an intake passage or an exhaust port by being seated on a valve seat of a cylinder head to which the intake, passage or the exhaust port is connected.
Normally, in an internal combustion, the higher the internal temperature of a combustion chamber, the higher the combustion efficiency. Heat in the combustion chamber is dissipated to the outside through the poppet valve in many cases. Therefore, a space is formed on or near a head surface of the poppet valve which comes into contact with the combustion chamber, and by forming a vacuum in this space, filling an inert gas, or filling a material having lower heat conductivity than a material forming the poppet valve, a heat insulating space is formed to suppress dissipation of heat inside, the combustion chamber (refer to Patent Literature 1).
By thus forming the heat insulating space, the temperature inside the combustion chamber becomes high, and if the temperature inside the combustion chamber is excessively high, knocking occurs and a predetermined engine output is not obtained, resulting in deterioration of fuel efficiency (deterioration of engine performance). Therefore, to lower the temperature inside, the combustion chamber, as a method for positively conducting heat (generated in the combustion chamber is a valve (method for increasing the heat dissipation effect of the valve), various hollow valves having a hollow portion loaded with a coolant together with an inert gas have been proposed.
In the poppet valve described in Patent Literature 2, a hollow portion is formed from the head portion to the stem portion, and this hollow portion is loaded with a coolant with heat conductivity higher than that of the base material of an engine valve (for example, metallic sodium whose melting point is approx. 98° C.) together with an inert gas.
The hollow portion of the engine valve extends from the inside of the head portion to the inside of the stem portion, and the amount of coolant to be loaded in the hollow portion is accordingly increased, so that the heat conductivity of the engine valve (hereinafter, referred to as heat dissipation effect of the valve) can be improved. However, the stem portion of the engine valve is reduced in volume since the follow portion is formed therein, so that if the heat dissipation effect is excessively great, the temperature of the stem portion excessively rises. An intake valve of engine valves takes-in a mixture of gasoline and air, and if the surface temperature of the intake valve is excessively high, the mixture to come into contact with this intake valve expands in volume, and the amount of mixture that can be taken-in in one cycle decrease, so that engine efficiency is deteriorated. In addition, the exhaust valve of the engine valves is exposed to a high temperature as compared with the intake valve, and in particular, a neck portion easily deteriorates in strength.